The invention relates to an installation for eliminating impurities during casting of metals, particularly impurities which float in the form of scum on the surface of the cast metal and more particularly comprise oxides of the metal. The invention is particularly applicable to casting of non-ferrous metals such as zinc.
All metals leaving furnaces in a molten state contain included impurities which generally consist of oxidized particles of the metal in question.
When the metal is poured into an ingot mold and left at rest, the impurities rise to the surface and float thereon in the form of scum. Irrespective of these impurities, quantities of oxides of the metal occur during the operations of casting it, depending on the manner in which the cast metal is handled. The quantity of oxides increases or decreases with the speed of casting. If the casting speed is constant, the proportion of oxides in the form of scum will still be increased or reduced depending on the height from which the metal cascades or falls on to the ingot mold.
After solidifying, the scum remains on the ingots, contaminating their metal. The scum also acquires a spongy texture into which ambient moisture, rain water and the like penetrate, making it very dangerous to re-melt the ingots subsequently.
Due mainly to these disadvantages, it is desirable to obtain ingots free from such scum and consequently free from the resulting disadvantages. It is therefore necessary to eliminate the impurities originating from the melting furnace or formed during handling of the cast metal.
For the reasons stated, these kind of impurities are usually eliminated on the ingot mold once the cast metal has been poured into it. This avoids the risk of further oxidation when the metal is left at rest.
Impurities in the cast metal poured into the mold are removed by means of scoops which move over the surface of the cast metal, starting from one of the edges, in order to pull along and collect the floating scum. Generally, two scoops are used and the scum is collected between them. The scoops can be actuated manually or mechanically. In the first case, the operation is laborious and dangerous, whereas the second case involves investment in machinery, maintenance costs and the like.
To solve there problems, Spanish Patent 466 025 by the present Applicants discloses a machine for separation of scum during casting of metals wherein the machine performs two separation phases or steps, both based on retention of the scum during the travel of the cast metal from the furnace to the mold. The first step or phase, during which the scum is retained, occurs in the casting ladle, where the cast metal arrives from the furnace. To this end, a siphon is formed at the outlet of the ladle and the cast metal flows through it so as to retain the scum floating on the metal. Between the casting ladle and the ingot mold, the cast metal flows along a conduit which discharges on to the mold, agitating the metal and thus resulting in further quantities of oxides, which are retained during a second or separation phase using a scum-retaining pan which is situated on the bottom of the mold and above which the outlet duct from the casting ladle discharges. The pan is formed with outlet orifices which open directly on to the bottom of the mold, thus avoiding further formation of oxides.
The machine described can efficiently retain the impurities which float on the cast metal, but there is a need for independent components, i.e. the casting ladle and retaining pan, and the corresponding actuating mechanisms, which have to be accurately coordinated with one another.
On the one hand, the casting ladle has to be mounted above a mechanism for tilting it between two extreme positions, i.e. a front or filling position at which it receives the molten metal from the melting furnace and at which the outlet of the siphon is situated at a height above that reached by the metal inside the mold, and a pouring position at which the outlet mouth of the siphon descends to a height at which the molten metal can flow out in sufficient volume to fill one or more ingot molds. At the same time, the scum-retaining pan must be mounted in a mechanism for moving it vertically between a bottom position, at which it rests on the bottom of the mold so as to receive the cast metal coming from the casting ladle, and a top position at which it is situated above the edge of the ingot mold wall so that the molds can be moved.